We investigated whether fusogenic lipid-surfactant mixed micelles (MM), a solubilized lipid system, composed of linoleic acid (LA) and polyoxyethylated (60mol) hydrogenated castor oil (HCO60) as a nontoxic solubilizer, could potentiate the antitumor activity of bleomycin (BLM) against inherently BLM-resistant murine L1210 leukemia. In a Winn-type tumor-neutralizing assay in which mice were inoculated intraperitoneally with L1210 leukemia cells preincubated with test materials, treatment with BLM plus LA-HCO60 MM showed a significant prolongation of mouse survival time as compared with BLM alone or MM alone. Further, combined use of BLM and concentrated MM accomplished total tumor cell killing, and all animals survived tumor-free. From these results, we presume that MM can make BLM-resistant tumor cells sensitive to BLM.